1. Field of the Invention
In one aspect, this invention relates to heat transfer elements useful in hot gas containment. In a further aspect, this invention is related to heater heads for use in hot gas engines.
2. Description of the Prior Art
Present hot gas engines, especially Stirling Engines, commonly have a heater head comprising a plurality of small diameter metal heater pipes formed into a complex array. The pipes are brazed or welded to form a closed loop. The array of heater pipes is filled with a pressurized working gas maintained at a high pressure, e.g., 150-200 atmospheres.
During operation, combustion gases are passed over the heater pipes; a portion of the combustion gases' heat is transferred to the working gas by conduction through the metal pipe. Alternate heating and cooling of the working gas drives a power piston.
The combustion gases in a combustion engine may be highly oxidizing because of the large amounts of oxygen and carbon monoxide present. The combustion gases also contain sulfur oxides and heavy metal oxides. Thus, the heater pipe surface exposed to the combustion gases must be resistant to oxidation and corrosive attack by oxygen sulfur and oxides.
The pressure of the working gas requires that the heater pipes also withstand high internal pressures at elevated temperatures, e.g., 1300.degree.-1400.degree. F (about 705.degree.-760.degree. C). The combination of high pressure and heat require a strong creep resistant material to contain the working gas.
Also, the pipe must retain the gas at the working temperatures and pressures. This requires that the pipe material have a low gas permeability. Because helium and hydrogen are generally used as the working gases, the heater pipe must be relatively dense and impermeable. Hydrogen is the preferred gas and therefore, it is desirable that the pipe be impermeable to hydrogen.
The combination of oxidizing atmosphere and high creep conditions severely limits the possible materials and operating temperatures of present engines. For example, stainless steel which has good oxidation and corrosion resistance has a high creep rate when operated continuously at 650.degree. C and 100 atmospheres pressure. Molybdenum and tungsten maintain their strength and creep resistance at high temperatures up to 1500.degree. C or more but are rapidly attacked by oxygen and oxides making their useful life very short.
Because of such material limitations, the prior art heater pipes could only operate at low operating temperatures of 1300.degree.-1400.degree. F and had a low heat transfer coefficient.
Because of their low heat transfer characteristics many heater pipes were needed in prior art systems. There were often several dozen heater pipes per cylinder. Assembling the heater head required the forming and sealing of the dozens of heater pipes to form a substantially hydrogen impermeable structure. The processing costs of forming such a heater head are substantial.
The more efficient operation possible with applicant's heater pipe provides a heater head with fewer heater pipes and results in a less complex structure. Also, the less complex structure is less expensive to assemble.